Question For Gilley
 

Hi Gilley-

I have been browsing these forums for a while, you seem to be the guru of tech. advice & I need some help. My 2000 ML55 has had a rapping noise at cold startup since 20K miles (45K now). Mercedes has replaced rocker arms, lifters (twice), and about 1 year ago diagnosed the problem as carbon buildup. They did a carbon cleaning which took care of the issue for about 5 months and it came back. They then had mercedes regional tech look at it and the carbon was back. They told me that it was the gas I was using. I only use 93 octane gas, mostly shell V power. My commute is 35 miles each way all highway at 80mph, so short of jet fuel Im not sure what else I can put in the car. They gave me Techron additive and told me to use it every 4th tank or so, which I have. The rapping is now back again. I'm not sure what else to do about it.

Have you ever heard of a carbon buildup issue like this? Any suggestions you (or anyone) have would be greatly appreciated, as to how I can get rid of this issue, I am at my wits end with it. Thanks.

I don't have an answer for this frustrating problem, but would suggest you might want to edit it and use a more descriptive subject line -- many of us tend to skip over non-descript subjects if pressed for time. Try something like "Rapping noise at startup -- 2000 ML55" and I bet you'll get some good advice.

Carbon build-up can indeed cause the symptoms you describe. I've had dozens of cars in this winter with the same cold-start/noise issue you describe. I normally use GM Top Engine Cleaner to resolve these complaints. I've got a 2002 E55 in the shop right now with the same problem, although this vehicle is constantly used for short trips. We inspected the pistons with a borescope and found heavy carbon deposits. We decided to try Techron so we added some to the fuel tank at about three times the recommended amount and drove the car around for a few days. It made an immediate difference. Inspecting the pistons again showed the carbon deposits were nearly gone. I can't explain the reason why your ML has an on-going problem, but at least the fix is simple and cheap. If you decide to use the GM product make sure you change the oil and filter afterward. Good luck.

I replied to this already yesterday, maybe at the ML forum? My thought was of the bulletin for the early ML55's (or maybe all early AMG 5.5L) about the headgasket problem. Mike this ring a bell? How many miles on it, and what is your history with the truck?

Gilly (no "E")

Hi Gilly, how's things? I did a search on Netstar this morning and didn't find anything about head gaskets OR carbon build-up. I'll look again this afternoon after things start to wind down for the day.

Ongoing carbon problem
 

Gilly- Sorry about the "E" - I have owned the truck since 5K miles. The problem has been recurring since about 18K. MB replaced rocker arms, lifters twice and has done full carbon cleaning. Their best explanation for me was the brand of gas I use, but as I stated prev., I only use 93 or higher octane, mostly Shell. I'm not sure what else to do to correct, this, and I get the impression that MB doesn;t either. Any suggestions are much appreciated.

tech bulletin
 

would an incorrectly profiled head gasket cause carbon buildup?

No, the point is the piston actually taps against the headgasket, not carbon. No idea why it would come and go unless it has to do with heat expanding the affected parts slightly, such as the rods.
Gilly